Bacteriorhodopsin represents the simplest, and possibly most abundant, phototropic system requiring only a retinal-bound transmem-brane protein to convert photons of light to an energy-generating proton gradient. The creation and interrogation of a m ...

Bacteriorhodopsin represents the simplest, and possibly most abundant, phototropic system requiring only a retinal-bound transmem-brane protein to convert photons of light to an energy-generating proton gradient. The creation and interrogation of a microbial rhodop-sin mimic, based on an orthogonal protein system, would illuminate the design elements required to generate new photoactive proteins with novel function. Described is a microbial rhodopsin mimic, created using a small soluble protein as template, that specifically photo-isomerizes all-trans to 13-cis retinal followed by thermal relaxation to the all-trans isomer, mimicking the bacteriorhodopsin photocycle, in a single crystal. The key element for selective isomerization is a tuned steric interaction between chromophore and protein, similar to that seen in the microbial rhodopsins. It is further demonstrated that a single mutation converts the system into a protein photo-switch without chromophore photoisomerization or conformational change.